Not Applicable
Not Applicable
1. Technical Field
This invention relates in general to integrated circuits and, more particularly, to variable oscillators.
2. Description of the Related Art
A variable oscillator, often referred to as a voltage controlled oscillator or VCO, is a circuit that produces a frequency within a specified range responsive to an input signal. Typically, the output frequency of the variable oscillator is controlled by the voltage of the input signal. In one common design for a voltage controlled oscillator, a varactor is coupled to a resonator (typically, a crystal) to produce a frequency which varies with the capacitance of the varactor.
Variable oscillators are used in a number of applications. One important application is communications, where variable oscillators are used for tuning to a specific channel in a specified frequency range. In many cases, a cellular phone circuit will use two or more variable oscillators for tuning.
Variable oscillators are one of the more expensive components in a cellular phone circuit. While many cellular phone functions are now integrated in a single chip, variable oscillators must remain outside the chip. This results in a larger and more expensive circuit. Further, currently available variable oscillators are controlled by an analog voltage signal. As a result, digital information from the DSP (digital signal processor) must be converted to an analog signal in order to control the varactor of the variable oscillator.
Therefore, a need has arisen for a variable oscillator which can be digitally controlled and which is subject to greater integration on a semiconductor circuit.
In the present invention, an oscillator comprises a resonator having first and second terminals and first and second capacitor banks coupled to said first and second terminals, respectively. The first and second capacitor banks each comprise a plurality of capacitors coupled to the resonator through respective switching devices that may be selectively enabled. The switches are selectively enabled to couple a desired set of said capacitors to said resonator. At least one of the switches is controlled with a clock signal having a programmable duty cycle to enable at least one of said capacitors during a first phase of the clock signal and disable that capacitor during a second phase of the clock signal.
The present invention provides significant advantages over the prior art. First, the oscillator may be numerically controlled rather than voltage controlled. Second, the oscillator may be precisely tuned by adjusting the duty cycle of the signal that controls one of the switches. Third, with the exception of the crystal, all other components of the oscillator can be fabricated onto a semiconductor circuit, reducing the part count needed for many devices, such as cellular phones.